1. Field of the Invention
The present invention relates to calibration of wireless modems.
2. State of the Art
Wireless data and voice connectivity is increasingly in demand. Wireless data connectivity is typically achieved using a wireless modem/radio combination. This combination is often referred to generically as a wireless modem.
Wireless modems exist for various radio transmissions standards. One such standard is the CDPD, or Cellular Digital Packet Data, standard. The CDPD service is implemented as an overlay on top of the AMPS, or Advanced Mobile Phone System, standard for cellular telephone communications.
Various advantages may be achieved by realizing a wireless modem as a separable modem and separable mobile radio device. Both the mobile radio device and the modem may be realized in the form of PC Cards as described in greater detail in U.S. patent application Ser. No. 08/724,459 (Attorney's Docket No. 024938-025) entitled TWO-PIECE PCMCIA MULTI-MODE WIRELESS MODEM, filed on Oct. 1, 1996, still pending, and incorporated herein by reference.
Although many advantages accrue as a result of the separation of radio and modem as described in the aforementioned application, the radio and modem must nevertheless operate in a well-coordinated manner in order to ensure compliance with the applicable radio transmission standard. In the case of the CDPD standard, which uses frequency modulation, the specification for modulation deviation is quite stringent. Modulation deviation refers to the deviation in frequency of the transmitted signal between when a logic 1 is being transmitted and when a logic zero is being transmitted. If the standard for modulation deviation is not strictly adhered to, then transmissions by the radio may not be correctly received.
Modulation deviation is determined by the combined operation of the modem and radio. In particular, the modem produces an analog modulation signal which is then applied to the radio. In response to this modulation signal, the radio varies the modulation deviation of the transmitted signal.
Because of variations in the manufacturing process, each radio exhibits a slightly different sensitivity, measured in volts/kilohertz, to the modulation signal input from the modem. For example, if the radio transmission standard calls for a deviation of 4.8 kHz (.+-.5%), one radio may give the required deviation in response to a modulation signal of 1.0V, and the next radio may give the required deviation in response to a modulation signal of 0.94V. However the modem is likewise subject to manufacturing variations. Hence, while the modem may be outputting a modulation signal that is nominally 1.0V peak-to-peak, in actuality, the modulation signal may be 1.1V peak-to-peak.
In the prior art, because most wireless modems are integrated combinations in which the modem and radio are not separable, the foregoing difficulty is easily overcome by performing a single factory calibration on the unit. As a result of the calibration operation, a calibration factor is then stored in non-volatile memory within the unit. During power-up initialization of the unit, this calibration factor is retrieved and used in order to achieve the proper modulation deviation.
In the case of a two-piece wireless modem of the type aforementioned, although the same type of calibration procedure may be applied to the combination of a particular modem and radio, requiring that the particular modem and radio always be paired together would defeat the purpose of separating the modem and radio and would negate the advantages that accrue from such separation. Of course, once the modem and radio are separated and paired with another modem or radio, as the case may be, the calibration results obtained from the original pairing are no longer valid.
One solution would be to separately and individually calibrate all modems and all radios. The cost of calibration, however, is high. One minute of test time may cost as much as the production cost of an entire modem. What is needed, then, is a calibration mechanism that allows a relatively stringent radio transmission standard to be reliably met, without requiring all modems and all radios to be separately and individually calibrated. The present invention addresses this need.